I used it for the first time today, spending 15 mins making a complex mask, only to discover when I wanted to back out part of it that there was no way back other than trying to replicate a gradient using flow and density. I honestly cannot understand how or why this feature wasn't shipped.

I
quite often use a gradient filter on a portrait to intensify or
transform subtly some backlight into a something more like a sunflare.
When I do this I have to erase some of the gradient falling on my
subject. The problem comes when I want to finetune the transition
especially on the hairs where I want to keep some of the initial filter.
It would have been logical to be able to restore it with the add filter
brush, unmasking the gradient according to the intensity of the applied Add Brush stroke .

Instead of this logical behaviour the Add Filter Brush behaves exactly like the adjustment brush, editing directly the filter, this is crazy! If someone want to add an area that falls out of the original filter
range, there is the adjustment/classic brush that is the right tool for
this purpose!

I also honestly cannot understand how or why this feature wasn't shipped.